The extensive use of thermoplastic materials in packaging and in other fields has resulted in widespread use of so-called "hot wire" devices to cut these materials, and to weld them together. Frequently the cutting and the welding are combined in one operation, in which overlaid thin sheets are welded together to form a seam along the line in which they are cut. The welding results from the elevation of the temperature by the hot wire to a point at which the material is plasticized or liquefied.
The temperature of the hot wire is maintained by treating it as an electrical resistance element. It is important that this temperature be controlled within relatively narrow limits, in order to assure that the temperature is both high enough to form the cutting and sealing effectively, and yet low enough to avoid burning the material. The latter condition usually results in accumulations of the burned material on the heating element, and interferes with its functioning. The usual arrangement for controlling the temperature of the hot wire is through monitoring the expanded length of the wire by causing it to actuate appropriately-placed switching devices. Present standard practice is to provide off-on conditions of the current traversing the hot wire corresponding to the peak and threshold temperatures as determined by the expanded length of the element. This procedure produces a number of problems. One of these is the inevitable trial-and-error procedure that becomes necessary in calibrating any new heating element, which must necessarily be periodically replaced as a result of wear and accident. Another problem with this form of control is the necessity for readjustment as the heating element accumulates residue from the sealed materials. A third problem is the obvious interference with monitoring the expanded length, as a result of lateral pressure on the heating element during the sealing and cutting process. It is also obvious that these same problems make it very difficult to determine and maintain an appropriate dwell time at which the sealing wire is held in contact with the sealed materials.